Desulfurization of distillate fuels



Patented Dec. 23, 1952 V2,623,004 DESULFURIZATION 0F DISTILLATE FUELS Harold 'Shalit, East Chicago, 11nd., .assigner tto Standard aOil Company, Chicago, Ill., a corporation .of` Indiana ApplcationJuly 22 1949, `Serial No. 106,292

2 lClaims. 1

This invention relates `to desulfurization of hydrocarbons of the distillatefuelboiling range and it pertains more particularly to an improved process employing 'a dialiphatic formamide solvent, such as N,Ndimethyl vformarnide wherein va high boiling, sulfur-containing charging stock 'is rst treated to produce a gas oil having a Llarger proportion of varomatic sulfur ,compounds to aliphatic sulfur compounds than were present in the original charge, `and said gas `oil is then extracted with 4said solvent iin an improved extracticn system.

`The production of high quality distillate fuels, such as diesel fuels, furnace oils, heater oils, etc., from high sulfur charging stocks of the gas oil boiling range presents a problem which is Very diiferent vfrom that of producing Alubricating oils from heavier charging stocks .or extracting aromatics from lighter charging stocks. Conventional lubricating `oil Aextraction solvents, `such as phenol, dichlorethyl ether, nitrobenzene, etc., give such poor'performance When'used for ,gas oil extraction that they are .unsuited for such purpose, and extensive/research has been conducted by the petroleum refining ,industry on Vthe problem .of preparing .high quality distillate fuels from high sulfur gas oils. An object of my invention is to provide a solution for thatproblem.

Another object is to `provide an limproved method and means for refining lhigh sulfur .charging stocks in a 'multi-step process wherein .the first step produces .a -gas .oil boiling .range material containing an enhanced proportion of aromatic sulfur compounds andthe second vstep .removes most of ,said sulfur compounds. ,An im# portant object is to provide arl/improved 4method and means for producing vhigh quality distillate fuels from 'the vso-called cycle `gas Oils which .are produced as ,by-products in thermal andcatalytic cracking processes. ,A further object yof the .invention is to provide .an improved method and means for obtaining a maximum yield ,of a .raffinate of a Vlow sulfur content 2in a .process for extracting hydrocarbons of .the `gas oil boiling range with ,dimethyl formamide and added water. vOther objects Will'be apparent as the detailed description of .the Ainvention proceeds.

While dimethyl formamide .is known to be a selective solvent for the naphthenic v.components .of lubricating oil charging stocks (note U. S. 2,166,140 and 2,183,852), it has not been considered useful as a desulfurizing agent. It apparently effects no substantial amount of desulfur-ization when applied to lubricating oils, and .even when applied to high sulfur virgin `gas voils, it effects removal vof only about 25% oi the sulfur. I have discovered, however, that dimethyl formamide will remove as'much as 80% t0'90% of aromatic sulfur compounds (eventhough it removes only about 25% to 35% of the aliphatic vsulfur compounds) of `the gas oil boiling range. When a high sulfur virgin gas oil is subjected to either thermal or catalytic cracking, there is a marked conversion of certain of the sulfur compounds contained in said charging stock so that While the original charging stock contains predominantly aliphatic sulfur compounds, the cycle gas oil conw tains predominantly aromatic sulfur compounds. Cracking thus produces a cycle gas oil containing sulfur compounds in a form amenable to separation by extraction withdimethyl formamide and the combination'of steps results in the production of distillate'fuels-ofremarkably high quality,

Whilehydrogen fluoride is recognized as oneV of the best available desulfurizing agents, I have found that a hydrogen fluoride raffinate of West Texas gas oil can be further desulfurized with dimethyl IOrmamide, apparently because the latter 4possesses the remarkable ability to remove `aromatic sulfur compounds,

In the dimethyl jformamide extraction step itself, I have Vfound that the :injection of .water .or aqueous solvent into the extract increases raf- :frnate yields Without materially decreasing ,desulfurization if the amount of water introduced is suiliciently small, e. g., is in the range of about 1% Ato '15 To avoid emulsification difficulties, the Watenif used :in appreciable quantities, should be introduced into the extract material as aque- `ous solvent mixture.

'The invention will be more clearly understood from the following detailed description read in con-junction vwith the accompanying drawing which forms a part of 4this specication and which is a process flow chart illustrating the seduenceof steps employed.

Before proceeding'wth the detailed description of the invention, it may be wellto point out the required characteristics of a high quality distillate fuel. Distillate fuels should boil chiefly in the range of about 400 F. to '650 F.; in other words, they are essentially of the lgas oil boiling range. It is important that distillate fuels have a low sulfur content, i. e., a sulfur content below about .5 per cent. Whenvemployed as diesel fuel, they should have a cetane number of at least about 40 and preferably labout 50 or more. An important characteristic of a distillate fuel (particularly furnace oils, 'heateroils Vand kthe like) is its freedom from coke and soot depositsj this 3 characteristic is rated by burning quality index or BQI and a high quality fuel should have a BQI of at least about 60 for most purposes.

The BQI as an index of distillate fuel quality is comparable to "octane number as an index for gasoline quality. The BQI of a. distillate fuel is determined by a standard burner test in a Coleman model 821B heater. The test involves determining the weight of total burner deposits when 100 gallons of fuel are burned at a rate of .3 gallon per hour employing a draft of 0.06 inch of water. A theoretically perfect fuel would give no burner deposits; such a fuel would arbitrarily be defined as having a BQI of 100. The BQI of any actual fuel is determined by subtracting from 100 the number of grams of burner deposits which accumulate under the above test conditions. Any fuel which deposits more than 40 grams of deposit (and hence has a '.BQI lower than about 60) does not meet the BQI requirements of a high quality distillate fuel, even though it may be a satisfactory fuel for many purposes.

While dimethyl formamide is the preferred solvent, it should be understood that other di-aliphatic formamide solvents, such, for example, as methyl ethyl formamide, diethyl formamide, etc., may be used. Dimethyl formamide is a liquid boiling at 1530 C. (approximately 308 FJ, it is heat stable, will not polymerize or form tars when heated, is non-corrosive, and is completely miscible with water. In addition, its density of 0.953 gm./1nl. is such that it is easily separated from petroleum stocks.

Referring to the drawing, a high sulfur charge oil from source I may be any reduced crude or gas oil which has a sulfur content of more than one per cent. In this example, the charge oil may be a West Texas crude or high boiling fraction thereof. This high boiling fraction may be subjected to coking or thermal cracking in any known manner in cracking system II to produce products lighter and heavier than gas oil and to produce a fraction of gas oil boiling range commonly referred to as cycle gas oil (which expression is intended to include coke still gas oil). Alternatively, a West Texas reduced crude or West Texas virgin gas oil may be cracked in system I2 which represents any conventional catalytic cracking process of the fixed bed, moving bed, or fluid type employing solid catalyst of the natural or synthetic silica alumina or silica magnesia type. I-Iere again the gas oil boiling range product is conventionally called cycle gas oil. As above stated, the sulfur compounds in the crude oil or virgin gas oil are chiefly present in the form of aliphatic sulfur compounds. The cracking step apparently alters these compounds to a considerable extent and the sulfur content of the cycle gas oils is chiefly in the form of aromatic sulfur compounds. At any rate, the proportion of aromatic to aliphatic sulfur compounds in gas oil boiling range materials is markedly increased by the cracking steps.

If the West Texas virgin gas oil is extracted with hydrogen fluoride for removal of sulfur in system I 3, the ranate produced apparently contains a considerable amount of aromatic sulfur compounds not removable by hydrogen fluoride. As above stated, even this so-called desulfurized gas oil can be further desulfurized by extraction with dimethyl formamide. It is also known to be an excellent charging stock for catalytic cracking.

The cycle gas oil (or HF raffinate) produced as hereinabove described and containing large amounts of aromatic sulfur compounds is introduced to dimethyl formamide extraction system I4. The extraction may be effected batchwise, in a multiple batch system, in a countercurrent batch system or in a countercurrent extraction tower, all of which extraction means are well known to those skilled in the art. I prefer to employ the type of extraction system which is conventionally used to extract lubricating oil with phenol and which is described, for example, in the National Petroleum News, Monthly Technical Section Devoted to Refinery Management and Petroleum Chemical Technology, section 2, December 6, 1944, pages R-830 to R840. In such a system, substantially anhydrous solvent is introduced at the top of a baffled countercurrent tower, oil charge is introduced at a level spaced from the bottom of the tower and water or aqueous solvent is introduced into the extract phase in the tower at about, but preferably below, the oil inlet level, raffinate being withdrawn from the top of the tower and extract from the bottom thereof.

Unless the amount of water or aqueous solvent introduced into the extract phase for increasing raffinate yield is held within a rather narrow range, there will be a substantial loss of desulfurization efficiency. With a solvent to oil ratio of the order 1:1, the amount of water introduced should not substantially exceed 5%, but amounts of water may be as high as 15% when sufficiently higher solvent to oil ratios are employed. A small amount of water may be introduced with the solvent at the top of the tower; this amount should not exceed about 1% to 2% since the water is most effectively utilized when introduced into the extract phase near` the bottom of the extraction tower. By per cent cf water introduced, I refer to the weight per cent of Water in the total solvent which is removed with the extract since some of this solvent may be introduced with the water at the base of the tower for avoiding emulsicaticn difiiculties. If large amounts of water are introduced at the base of the extraction tower, such water should contain an amount of solvent from about one-half to twice its volume in order to avoid objectionable emulsication. v

The extract from the base of the extraction tower is freed from solvent in solvent recovery system I5, which is preferably a conventional extract stripper which is operated at a suicient ly high bottom temperature to insure substantially complete removal of solvent. When water is employed, a separation between water and solvent may be effected in the top of the extract stripper or distillation column; however, a separate column may be employed for removing water from solvent. The cycle gas oil charging stock to the dimethyl formamide extraction system should not contain components lower boiling than about 350 F. because lower boiling components might accumulate in the solvent and thus require additional equipment for solvent recovery. The water or aqueous solvent recovered from system I5 is preferably returned to the lower or extract portion of the extraction system while the solvent is returned to the raffinate or upper part of the extraction system. An alternative method of solvent recovery is simply to wash the extract with water and subsequently remove the water from the resulting aqueous solution. A stripped raffinate and/or extract may be additionally Washed with water for re moving any small amounts of solvent which may be retained therein. Solvent is removed from the raffinate in solvent recovery system I 6 in the same general 4manner as described `in connection with solvent recovery system .l 5..

The final extract I1 is highly aromatic and is character-ized by a high sulfur content which may be of the order of 2% to 4% or more. The finished raffinate I8, on the v other hand, 'is char- 6 YHeretofore 'hydrogen fluoride has 'been -considered to be the lmost outstanding and best solvent for effecting desulfurization of hydrocarbon oils. The following results show the improvement that can be obtained by extracting an HF raffinate of West Texas gas oil and dimethyl formamide:

Y Percent i Percent Percent- I Sp. Cetane -Yieia s Desuif. D4" L "nl Disp. No.

HF-Raffnate of W. T. Gas Oil... 0.51 0.846 1. 4748 123 57 Dimethyl formamide Raiinate..Y 83. 4 0. 38 22.4 0.839 1. 4641 106 02 acterized by a sulfur content below .5 weight per cent and is characterized by a remarkably high BQI and usually by a high cetane number. This finished raffinate may be further distilled to meet any specific distillate fuel specifications, and the resulting distillate fuel is of remarkably high quality. The finished raffinate is also exceptionally Well suited as a catalytic cracking charging stock.

As previously pointed out, the invention is not limited -to extraction in a countercurrent tower and tliedata set forth in the following examples were obtained by batch extraction runs. In any case, the solvent to oil charging stock ratio should usually be in the range of to 150 volumes or more of solvent per 100 volumes of charging stock. Extraction temperatures should be in the range of about F. to 140 F. and preferably from about F. to 120" F. at approximately atmospheric pressure.

When a West Texas virgin gas oil was extracted with 'fan equal volume of dimethyl formamide at 77 F., the results were as follows:

l5 .The HF rafnate itself was too high in sulfur to meet the .5 per cent requirement, but re-ex- Ytraction of this raffinate with dimethyl formamide lowered the sulfur content to .38 per cent, lowered the refractive index to 1.4641 and increased the 20 cetane number to 62.

l As above stated, the raflinate obtained by extracting a thermal or cata-lytic cracking cycle oil with dimethyl formamide is also exceptionally well suited as a catalytic cracking charging stock. TIfhe solvent apparently extracts from the cycle gas oils those substances which are most detrimental in catalytic cracking systems. The rafiinate thus produced, when charged to a fixed bed, moving bed, or fluid type catalytic cracking process employing natural or synthetic silica alumina catalysts of known composition, gives a remarkably high conversion to high octane number gasoline. It avoids catalyst deactivation which would otherwise be caused when high sulfur stocks are charged to a unit employing natural catalyst. It results in remarkably low losses to coke and gas vformation. The removal from cycle gas oils of Yield Percent Percent Sp. Cetane Percent Desulf. D4 ""20 Disp. No.

W. T. virgin gas oil 1. 4s o. 87o 1. 481s 127. 4 .so Ranate 78. 8 1. 10 25. 7 849 1. 4683 108 G1 Extract 21.2 3. 86 1. 5408 While a relatively high yield was obtained of a raffinate characterized by low refractive index and high cetane number, it will be noted that only about 25.7 per cent of the sulfur was removed, and that the ranate would fail to meet specications from this standpoint. However, when a catalytic cycle gas oil was extracted in the same Way with an equal volume of dimethyl formamide at 77 F., the following results were obtained:

components which are so deleterious to distillate fuels apparently results in a catalytic cracking charging stock which is superior even to virgin gas oil.

I claim:

1. The method of producing a high quality distillate fuel, which method comprises cracking a virgin gas oil which contains more than 1 percent sulfur and in which the sulfur content is chiefly in the form. of aliphatic sulfur compounds to Percent Percent Percent 2 D E p. Cetane Yield Desulf. D* a '"'Dz Disp. No. BQI

Lt. Cat. Cycle Oil l. l5 0. 872 1.4967 154 1 35 20 Rainate 0. 39 66. l 0. 835 l. 4680 122 49 8l l. 96 0.945 1. 5365 1 Calculated.

Here it will be noted that in addition to obtaining a Araffinate of 1.4680 refractive index, the raffinate sulfur content was reduced to .39, the cetane number was approximately 50, and the BQI was approximately 80. It is most remarkable that the per cent desulfurization on this catalytic cycle gas oil was approximately three times that obtained on virgin gas oil, and it is most unique that any solvent could produce from by-product catalytic cycle gas oil a product having the desirable distillate fuel characteristics which are exhibited by the raffinate in this case.

, convert sulfur compounds contained in the virgin gas oil and to produce a cracked product oil of i which the fraction boiling chiefly in the range of r400' F. to 650 F. contains at least about .5% by weight of sulfur which is chiefly in the form of aromatic sulfur compounds, extracting said product oil fraction which boils chiefly in the range of 400 F. to 650 F. and which contains sulfur chiefly in the form of aromatic sulfur compounds produced in the cracking step with dimethyl formamide at a temperature in the range of 35 to F. with a solvent to charging stock ratio 7 in the range of .2:1 -to 1.5:1 to effect formation A REFERENCES ,C I'IJED 0f Separate extract and raffinate Phases separat' The following references are of` record in the ing the rainate phase from the extract phase meof this patent: and recovering dirnethylformamide solvent `from UNI STATES PATENTS v 1 each of the separated phases. 5

2. The method of claim 1 which includes the Number Name Date further step of employing in the extraction step 2,162,963 MCKttriCk June 20, 1939 an amount of Water in the range of about 1 to 15 2,166,140 Hansley July 18, 1939 volume percent based on total solvent mixture 2,183,852 Boyd Dec. 19, 1939 in the extract phase. 10 2,342,888 Nysewander et a1. Feb. 29, 1944 HAROLDv SHALIT. 2,386,927` BOyd Oct. 16, 1945 

1. THE METHOD OF PRODUCING A HIGH QUALITY DISTILLATE FUEL, WHICH METHOD COMPRISES CRACKING A VIRGIN GAS OIL WHICH CONTAINS MORE THAN 1 PERCENT SULFUR AND IN WHICH THE SULFUR CONTENTS IS CHIEFLY IN THE FORM OF ALIPHATIC SULFUR COMPOUNDS TO CONVERT SULFUR COMPOUNDS CONTAINED IN THE VIRGIN GAS OIL AND TO PRODUCE A CRACKED PRODUCT OIL OF WHICH THE FRACTION BOILING CHIELFLY IN THE RANGE OF 400* F. TO 650* F. CONTAINS AT LEAST ABOUT .5% BY WEIGHT OF SULFUR WHICH IS CHIEFLY IN THE FORM OF AROMATIC SULFUR COMPOUNDS, EXTRACTING SAID PRODUCT OIL FRACTION WHICH BOILS CHIEFLY IN THE RANGE OF 400* F. TO 650* F. AND WHICH CONTAINS SULFUR CHIEFLY IN THE FORM OF AROMATIC SULFUR COMPOUNDS PRODUCED IN THE CRACKING STEP WITH DIMETHYL FORMAMIDE AT A TEMPERATURE IN THE RANGE OF 35* TO 140* F. WITH A SOLVENT TO CHARGING STOCK RATIO IN THE RANGE OF .2:1 TO 1.5:1 TO EFFECT FORMATION OF SEPARATE EXTRACT AND RAFFINATE PHASES, SEPARATING THE RAFFINATE PHASE FROM THE EXTRACT PHASE AND RECOVERING DIMETHYLFORMAMIDE SOLVENT FROM EACH OF THE SEPARATED PHASES. 